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Comparison between gamma-alumina and aluminum niobate supported vanadium oxides in propane oxidative dehydrogenation
V. Cork% Corbcrin and S. V I C Bellon (Edilors), New Developments in Selective Oxidalion If 0 1994 Elsevier Science B.V. All rights reserved.
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Comparison between gamma-alumina and aluminum niobate supported vanadium oxides in propane oxidative dehydrogenation J.-G. Eona, P. G. Pries de Oliveirab, F. Lefebvrec and J.-C. Voltac "Instituto de Quimica, Departamento de Quimica Inorganica and NUCAT/IJFRJ, Bloc0 A63 1, Ilha do Fundao, 21945-970 Rio de Janeiro, Brazil bInstituto Nacional de Tecnologia, Avenida Venezuela 82, Praca Maua 2008 1, Rio de Janeiro, Brazil %stitut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France The catalytic properties of vanadium oxides supported by alumina and aluminum niobium oxides (AINbO) are compared in propane oxidative dehydrogenation. VO,/AlNbO are less active than VOx/y-A1203, but more selective in the transformation into propene. For the first system, selectivity is maximum at an intermediate calcination temperature of the supports, corresponding to an amorphous AlNbO structure. Bridging oxygens (V-0-V) from the two dimensional V 0 4 array are suggested to be active sites on both catalysts. However, it is not possible to correlate the catalytic properties of superficial vanadium oxides only with their degree of condensation; it is suggested that the structure and the acid-base properties of the support also define the redox function of supported vanadates. 1. INTRODUCTION
The activation of the C-H bond in propane oxidative dehydrogenation is a challenging question in the fascinating route for the conversion of alkanes into chemical utilities. The nature of catalytic surface sites is important since it defines the reactivity of the corresponding systems. In the case of the magnesium vanadate catalysts, it has been shown by electrical conductivity measurements, that the 0 2 - entities are responsible for the oxidation of propane. The activity and selectivity of the different VMgO polyvanadates have been attributed to their respective degree of condensation (ortho-, pyro- and meta-)( 1,2). The structure, redox properties and reactivity of vanadium oxide surface compounds have previously been studied on different oxide supports such as TiO2, AI2O3, Z r 0 2 and Si02 (36). It has been shown that it is possible to change the dispersion of vanadia, depending on the nature of the support. The structure and the acid-base properties of the support were considered as the important parameters to control this dispersion. Recently, we have studied (7,s) the properties of aluminum-niobium oxides of composition Al:Nb:O = 1 : 1:4 (abbreviated as AlNbO in this paper) as supports for vanadium oxides. From FTIR and X P S studies of these
84
oxides, it had been suggested (7) that small areas of alumina (and maybe niobia) exist at the surface of the amorphous AlNbO support. It is thus interesting to compare the catalytic properties of catalysts VOx/y-A1203 and VOx/AINbO in order to clarifjl the role of the alumina areas in the latter. In this communication, we compare AlNbO supports to y-Al2O3, for vanadia dispersion and propane oxydehydrogenation . It is confirmed that the support determines the dispersion of superficial vanadates, but it is suggested that it also affects the redox properties of these oxide species. 2. EXPERIMENTAL Part of the results has already been published (7,8), therefore the experimental details will be abbreviated. y-Al2O3 supported vanadium oxides have been prepared following the continuous adsorption method. The superficial loading and the nature of the precursor species were monitored by varying the pH of the 0.05M ammonium vanadate impregnation solution. Three samples have been prepared at pH 7.0, 4.5 and 2.5, corresponding respectively to V30g3-, v 4 0 1 2 ~ - , V1oO2g6- and V10028H5- solution species (9), using a non-porous (200 m2g-I) y-alumina support. Aluminum niobate supported vanadium oxides have been prepared by grafting VOCl3 in inert atmosphere on AlNbO oxides calcinated at different temperatures. The AlNbO oxides have been prepared from oxalates precursors (7). All catalysts have been calcinated under air flow at 500OC. The catalysts have been tested with propane oxidative dehydrogenation in a flow system. The catalyst (100-350 mg) was deposited on a fixed bed in a quartz microreactor (Utube, 13 mm diameter) operating under atmospheric pressure. The gas mixture containing propane (2 vol %), 0 2 (19.6 vol %) and N2 (78.4 vol %) was fed at a flow rate of 50 ml/min. The conditions for on-line chromatographic analysis have been described elsewhere (8). The vanadium coordination was investigated at ambient conditions by UV-visible, Raman, V NMR and ESR spectroscopies after preparation and catalytic testing. In situ characterization of gamma-alumina supported vanadium oxides has also been performed in a Raman cell under dry air.
3. RESULTS 3.1. Characterization after calcination Table 1 summarizes the results of chemical analysis of the two families of catalysts. It must be noted that the AlNbO supports in samples AN500, AN600 and AN650 are amorphous to Xray diffraction. Sample AN750 corresponds to crystallized AlNbO4. It is seen that the vanadium superficial coverage on the four AlNbO supports varies within a narrow range (0.18 to 0.26), well below the values obtained on y-Al2O3 (0.28 to 0.65). This has been associated with the low density of surface hydroxyl groups on AlNbO oxides when compared to y-Al2O3.
85
UV-visible DRS spectra of the catalysts after calcination show a strong asymmetrical band in the 350-420 nm domain. The position of the band for all solids is reported in table 1. This range is characteristic of the ligand-metal charge transfer observed with V5+ ions. No signal is observed in the 700-800 nm zone, corresponding to the V4+ d-d transition. It is difficult, on the basis of UV experiments to assign a definite coordination to the superficial V species, as the spectrum is not related to that of bulk reference compounds with tetrahedral and octahedral coordination, Table 1 Comparison of some features of the two catalysts series Catalyst
BET area (m2g-1)
[V](at/nm2)
8
h,,,,
AN500 AN600 AN650 AN750
79 73 57 39
1.6 1.4 1.1 1.5
0.26 0.23 0.18 0.25
393 396 396 400
A2
200 200 200
3.9 3.3 1.7
0.65 0.54 0.28
420 3 80 350
A4 A7
G(ppm) -580, -548, -541, -486,
-1984 -1915 -1911 -1903 -502 -530 -544
AN: VO,/AlNbO. The associated number in the name indicates the calcination temperature of the support. A: VOxly-Al2O3. The number indicates the pH of the adsorption solution. [V]: vanadium superficial density. 8: vanadium superficial coverage (calculated from the unit area: A(VO,)=0.165 nm2 ). h,,,: position of UV-visible band. 6(pprn):S1V NMR chemical shift in reference to VOC13.
Table 1 shows also the 51V N M R chemical shift, in reference to VOC13, observed in the two families after calcination. On the basis of the anisotropy of the N M R signal, the sharp line observed (7) at -1900 ppm for the vanadium oxide species supported by AlNbO oxides has been attributed to an isolated vanadate with tetrahedral coordination. The wide, asymmetrical band, observed at approximately -500 ppm is associated with V species with distorted tetrahedral coordination. In contrast with the former, it is assumed that these species are condensed vanadates, forming linear chains or bidimensional arrays. It is seen that the proportion of the two kinds of tetrahedral vanadates (isolated or condensed) on AlNbO supports, depends on the calcination temperature of the material. A Raman scattering study was only possible for VOx/y-A1203 samples, because of a strong absorption by the Nb-0 vibration modes of the AlNbO support in the same region as the V-0 bands, As observed by other authors (lo), the nature of the alumina supported vanadates is influenced by the water pressure above the sample. The spectra have thus been collected at ambient temperature before and after in situ calcination (400OC under dry air). Only results after dehydration are shown in fig. 1.
86
.-a C
fa
800
1000 cm-1
1200
Figure 1. Raman spectra of VOx/y-A1203 (A2, A4 and A7) and alumina (A) A wide band is observed in the three samples A7, A4 and A2 in humid and dry conditions in the 960-995 cm-l domain; after drying, a shoulder develops above 1000 cm-l. This can be attributed to a V=O species forming three bonds with the support: this species is not stable in presence of water, forming hydroxylated, mono-oxo species which absorb in the same region as polycondensed vanadates (960-995 cm-1). W e therefore conclude that both types of vanadates (isolated and condensed mono-oxo vanadates) are present at the surface of the alumina support in the conditions of the catalytic reaction. The shift of the band to lower frequencies from A2 to A7, in hydrated samples, has been interpreted (1 1 ) as a decrease in lateral interactions between linear or branched vanadate chains as their superficial density decreases. 3.2. Propane oxydehydrogenation
Catalytic results for propane oxidative dehydrogenation at approximately isoconversion are shown in table 2.
Table 2 Catalytic results for propane oxydehydrogenation Catalyst
Mass (mg) T(0C) Conversion (“A) TON(s-1)
A2 A4 A7
100
100
100
350 350 350
13.5 8.8 <1.
A7
100
400
7.5
AN500 AN600 AN650 AN750
200 200 350 125
500 500 500 500
11.8 9.2 10.4 11.5
7 . 7 lo4 ~ 6 . 0 lo4 ~ <1.3 x lo4
2.1 x 2.0 10-3 2.1 x lo-’ 7.0 10-3
C?Hh
Selectivity(“) co CO:,
c
41 50.4
--
17.5 14 --
28 16
__
-_
58.4
12.1
--
70
12 65 62 44
87 2 16 18
__
99 100 99 87
33 21 25
87 80
C= selectivity sum (carbon balance); TON= turn over number (propane molecules transformed per vanadium atom and per second) The two families of catalysts present different behavior. VOx/y-A1203 catalysts are much more active than VO,/AlNbO catalysts so that it has not been possible to compare the two series at the same temperature. Results for sample A7 are shown at 400oC because practically no conversion is observed at 350oC. This sample is thus far less active than A4 or A2.This point is more obvious from their turnover numbers (TON). The propene selectivity pattern is however consistent for the three samples (1 1). VO,/AINbO catalysts on the other hand present a higher selectivity to propene at an intermediate calcination temperature of the support (AN600 and AN650). Sample AN500 shows a very low propene selectivity and a higher C 0 2 selectivity. Sample AN750 shows a selectivity compatible with alumina-supported vanadates; its activity (TON) is also the highest in the VOx/AINbO series. 3.3. Characterization of VO,/y-A1203 after catalytic testing
The UV-visible DRS spectra performed on alumina supported vanadium oxides show a new band at 700-800 nin which is attributed to the d-d transition of V4+ ions in octahedral symmetry. It is observed that the intensity of the band increases strongly with the vanadium coverage. On the other hand, the charge transfer band at 300-400 nm has not been significantly modified when compared with the calcinated catalyst. 51V NMR spectra of the same solids are also very similar before and after reaction. ESR spectra measured at ambient temperature show identical qualitative features for the three catalysts, but with different intensities. The ESR parameters are reported in table 3, where they are compared to the parameters of reduced amorphous V2O5 obtained from (12). A reasonable fit is observed between the two sets of parameter. It is interesting to note that the structure of amorphous V2O5 has been proposed to be built up from branched tetrahedrally coordinated mono-oxo vanadates (12).
88
Table 3 Spin Hamiltonian Parameters Spin parameters
VOX/y-Al203(after testing) 1.919 1.982 190 Gauss 70 Gauss
amorphous V205( 12) 1.913 1.985 176 Gauss 66 Gauss
4. DISCUSSION 4.1. VOx/y-A1203 UV-visible spectroscopy clearly establishes the presence of only V5+ species dispersed at the surface of the support after the calcination of the catalysts. l V N M R spectroscopy shows that the major part of vanadium ions are tetrahedrally coordinated and form linear chains or bidimensional arrays. Thus, the two techniques do not show any clear difference between the three samples. Raman spectroscopy indicates that mono-oxo polyvanadates are formed with a higher degree of lateral interaction as superficial coverage increases. Tables 1 and 2 show that the turnover number for these solids strongly increases with their vanadium content. This observation suggests that dispersed species are not very active sites for propane oxydehydrogenation. As indicated by Raman spectroscopy, the extent of lateral interaction between vanadate chains is the only factor differentiating the three catalysts after calcination so that higher activity of vanadate species might result from these interactions. UV-visible and 51V NMR spectroscopy show that little alteration occurs in the vanadium V structure after partial reduction. These results on one hand, and the reasonable similarity of the ESR spectra in reduced VOx/y-A1203 catalysts and reduced amorphous V2O5, on the other hand, lead us to suggest that the active site in propane oxidative dehydrogenation is part of a branched vanadate chain in which vanadium is tetrahedrally coordinated. 4.2. VOx/AINb04 Two different vanadium environments have been characterized by 51V N M R spectroscopy. The first one is preferentially observed when the AlNbO support is calcinated at low temperatures and is characteristic of condensed tetrahedrally coordinated vanadium sites observed on y-Al2O3. As the calcination temperature of the support is increased, this species is substituted by a more symmetrical one which has been assumed to correspond to an isolated vanadium site. Table 2 shows that the turnover number is approximately the same in the three samples, AN500, AN600 and AN650. AN750 is roughly three times more active. These results show that the symmetrical (isolated) vanadate on AlNbO4 is more active than the distorted (condensed) species on amorphous AlNbO oxides. Propene selectivity on the other hand is the
89
best at intermediate calcination temperature (i.e. amorphous AlNbO oxides with reduced superficial hydroxyl concentration). At lower calcination of the support ( A N S O O ) the catalyst gives very poor propene selectivity. The activity and selectivity pattern of these AlNbO supported vanadates is thus not clear. 4.3. Comparison of t h e two systems
As too many parameters are involved in the comparison of the two series (nature and surface area of the support, vanadium loading, reaction temperature) this will be restricted to the analysis of the selectivity differences. Indeed, the curve selectivity-conversion for VOx/yA203 catalysts is not significantly affected by vanadium loading and reaction temperature (1 1). Only two variables will thus be considered to analyze the catalytic results summed up in table 2. These are the degree of vanadium condensation and the chemical properties of the support. The vanadate species supported by amorphous AlNbO oxides may be compared to the alumina supported vanadates as their 51V NMR spectra are quite similar. W e have concluded for VOx/y-A1203 solids that an increase in lateral interactions leads to more active catalysts for propane oxidative dehydrogenation. Thus, it is not easy to understand, on this basis, why the new symmetrical (isolated) species, formed on the crystallized AlNbO4 support, should be more active than the distorted (condensed) one formed on amorphous AlNbO supports. We therefore must imagine that redox properties of the vanadate species depend on the bonds formed with the support. In a previous work (7), we have determined the acidity of the hydroxyl groups as a function of the calcination temperature of the support, by measuring their turnover number for isopropanol dehydration. It appeared that OH groups on crystallized AlNbO4 oxide are much more acidic than corresponding amorphous AlNbO-bonded groups. It therefore seems reasonable to admit that some field effect, induced by the linkage of niobium to aluminum sites, should also modify the electronic distribution in the V - 0 bond. It is obvious that some modification occurs from the 51V NMR spectrum: indeed the strong chemical shift (-1900 ppm) characterizes an unusual electronic density around the vanadium nucleus. It is also interesting to note that the basicity of the vanadium bonded oxygen, measured by oxidation of isopropanol to acetone, increases with the concentration of the new species. Also very striking is the lack of selectivity observed in sample AN500. If we assume that the vanadate species are the same as in the catalyst AN600, and VOx/y-A1203, as indicated by l V NMR spectroscopy, we must imagine that some influence from the support modifies the catalytic properties of the species. It might be possible that acidic functions of the support coupled with redox sites, for example, strongly influence the selectivity of the catalyst. In conclusion, the comparison between the two families of catalysts reported in this work suggests that the reactivity of supported vanadium oxides is determined by the degree of condensation of the vanadate species and the acid-base properties of the support. It seems that the challenge in controlling the catalyst reactivity will be mainly resolved when both steps, support preparation and vanadium dispersion, are strictly controlled to yield a strongly homogeneous and well defined superficial structure. REFERENCES 1. D.Siew Hew Sam, V.Soenen and J.C. Volta, J. Cata1.,123, 417 (1988) 2. A. Guerrero-Ruiz, I. Rodriguez-Ramos, J. L. G. Fierro, V. Soenen, J. M. Hermann and
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3. 4. 5.
6. 7.
8. 9. 10. 11. 12
J.C. Volta, in New Developments in Selective Oxidation by Heterogeneous Catalysis, in Surface Sciences and Catalysis, vo1.72, p203, P. Ruiz and B. Delmon Eds., Elsevier(l992) J.Haber, A.Kozlowska and R.Kozlowski, J.Catal., 102, 52 (1986) I.E.Wachs, J.Catal.,124,570(1990), U.Scharf, M. Schraml-Marth, A. Wokaun and A. Baiker, J. Chem. Soc. Faraday Trans, 8., 3299, (1991) J.M.Jehng and I.E.Wachs, Catal. lett.,13, 9, (1992) P.G.P.de Oliveira, F.Lefebvre, J.G.Eon and J.C.Volta, J. Chem. Soc., Chem. Com., 1480,2 1( 1990) P.G.P.de Oliveira, J.G.Eon and J.C.Volta, J.Cata1.,137, 257 (1992) M.T.Pope, Heteropoly and Isopoly Oxopolymetalates, Springer-Veda&( 1983) G.T.Went, S.T.Oyama and A.T.Bel1, J.Phys.Chem.,4240,94,(1990) , J.G.Eon, R.Olier and J.C.Volta, J. Catal., to be published A. Mosset, P. Lecante, J. Galy and J. Livage, Philos. Mag. (8) 46 (2), 137 (1982)
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J. EMBER (Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland): Some time ago we have shown that the type of vanadium oxide species present at the surface depends on the water pressure and coverage. In the humid atmosphere decavanadate anions are favored, whereas on careful outgasing either isolated V04 tetrahedra appear or chains of these tetrahedra forming a metavanadate type structure. Which of these exist in the conditions of your oxydehydrogenation and which is then an active phase?
J. G. EON (Instituto de Quimica, Rio de Janeiro, Brazil): We have shown (11) that decavanadate anions are not stable on the alumina surface. After calcination at 5OO0C, even in humid atmosphere, we have characterized mainly tetracoordinated species forming a chain or array structure. Isolated V04 species might coexist; as the specific activity increases greatly with vanadium concentration from A7 to A2, we suggest this species is not the active one on alumina supports. G. CENT1 (Dip Chimica Industriale e dei Materiali, V. Le Risorgimento , Bologna, Italy): In the samples prepared by supporting vanadium on alumina you observe an increase in the activity in the A7-A2 series that was attributed to an increase in the lateral interaction between vanadate species. However, these samples have a different vanadium content. My question is therefore how change the specific activity per vanadium site in these samples and how this specific activity can be correlated to the change of surface species of vanadium. J. G. EON: What we observe is indeed an increase in specific activity per vanadium site (turnover number) in the series A7, A4, A2. We suggest that lateral interactions between vanadate chains stabilize the superficial vanadyl formed after reduction by propane (1 1).
J. S. R U E (Inst. Catalisis, Madrid, Spain): The table with the ESR results is not well written. The g-values should appear as g,, and gL. They should be determined by computer simulation. The hyperfine splitting constants indicate that the sites are different, not similar and should also differentiate between the perpendicular and parallel components. J. G. EON: We agree with these comments. However, the ESR parameters of V4+ in alumina supported vanadates are very different from the values observed in crystallized V2O5 or VOPO4 and strikingly in reasonable agreement with those of amorphous V2O5. Although the sets are clearly not identical, we use the spectrum as a finger-print, suggesting that the structure of vanadate layers on alumina should be compared to that of amorphous V2O5 where vanadium is also tetracoordinated.
G. BUSCA (Instituto di Chimica, Faculta di Inorganica, Universita di Genova, Genova, Italy): In your lecture you denoted the active sites on VOx/A1203 as tetrahedral with Td symmetry. Previous data from several laboratories agree showing that active sites on vanadia-alumina in dry conditions are tetracoordinated but not tetrahedral. Vibrational spectroscopies in fact show that these species have one short V=O bond and probably, three
92
longer V - 0 bonds. They are consequently tetracoordinated, but with C3v symmetry. Tetrahedral centers (Td symmetry) are instead typical of vanadate species in basic environments. In your opinion, your data support a tetrahedral (Td) symmetry or a tetracoordinated symmetry (CjV)?
J. G. EON: We think it is impossible to attribute symmetry labels from our data to alumina supported vanadates. Our spectroscopic data show that these species are mono-0x0, tetracoordinated vanadates. J. F. BRAZDlL (BP Chemicals, Warrensville Research Center, Cleveland, Ohio, USA): Since aluminum niobate crystallizes with the rutile structure and vanadium is known to dissolve readily into rutile structures, does the isolated vanadium species you see arise from formation of solid solution between vanadium and aluminum niobate? J. G. EON: This is an interesting point. However, aluminum niobate does not crystallize with the rutile structure but is isomorphic to Ti02(B)(7). Moreover, vanadium dissolves in rutile as the reduced V4+ cation. Since we have observed only V5+ species by UV-visible spectroscopy, we did not consider this possibility.
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Comparison between gamma-alumina and aluminum niobate supported vanadium oxides in propane oxidative dehydrogenation J.-G. Eona, P. G. Pries de Oliveirab, F. Lefebvrec and J.-C. Voltac "Instituto de Quimica, Departamento de Quimica Inorganica and NUCAT/IJFRJ, Bloc0 A63 1, Ilha do Fundao, 21945-970 Rio de Janeiro, Brazil bInstituto Nacional de Tecnologia, Avenida Venezuela 82, Praca Maua 2008 1, Rio de Janeiro, Brazil %stitut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France The catalytic properties of vanadium oxides supported by alumina and aluminum niobium oxides (AINbO) are compared in propane oxidative dehydrogenation. VO,/AlNbO are less active than VOx/y-A1203, but more selective in the transformation into propene. For the first system, selectivity is maximum at an intermediate calcination temperature of the supports, corresponding to an amorphous AlNbO structure. Bridging oxygens (V-0-V) from the two dimensional V 0 4 array are suggested to be active sites on both catalysts. However, it is not possible to correlate the catalytic properties of superficial vanadium oxides only with their degree of condensation; it is suggested that the structure and the acid-base properties of the support also define the redox function of supported vanadates. 1. INTRODUCTION
The activation of the C-H bond in propane oxidative dehydrogenation is a challenging question in the fascinating route for the conversion of alkanes into chemical utilities. The nature of catalytic surface sites is important since it defines the reactivity of the corresponding systems. In the case of the magnesium vanadate catalysts, it has been shown by electrical conductivity measurements, that the 0 2 - entities are responsible for the oxidation of propane. The activity and selectivity of the different VMgO polyvanadates have been attributed to their respective degree of condensation (ortho-, pyro- and meta-)( 1,2). The structure, redox properties and reactivity of vanadium oxide surface compounds have previously been studied on different oxide supports such as TiO2, AI2O3, Z r 0 2 and Si02 (36). It has been shown that it is possible to change the dispersion of vanadia, depending on the nature of the support. The structure and the acid-base properties of the support were considered as the important parameters to control this dispersion. Recently, we have studied (7,s) the properties of aluminum-niobium oxides of composition Al:Nb:O = 1 : 1:4 (abbreviated as AlNbO in this paper) as supports for vanadium oxides. From FTIR and X P S studies of these
84
oxides, it had been suggested (7) that small areas of alumina (and maybe niobia) exist at the surface of the amorphous AlNbO support. It is thus interesting to compare the catalytic properties of catalysts VOx/y-A1203 and VOx/AINbO in order to clarifjl the role of the alumina areas in the latter. In this communication, we compare AlNbO supports to y-Al2O3, for vanadia dispersion and propane oxydehydrogenation . It is confirmed that the support determines the dispersion of superficial vanadates, but it is suggested that it also affects the redox properties of these oxide species. 2. EXPERIMENTAL Part of the results has already been published (7,8), therefore the experimental details will be abbreviated. y-Al2O3 supported vanadium oxides have been prepared following the continuous adsorption method. The superficial loading and the nature of the precursor species were monitored by varying the pH of the 0.05M ammonium vanadate impregnation solution. Three samples have been prepared at pH 7.0, 4.5 and 2.5, corresponding respectively to V30g3-, v 4 0 1 2 ~ - , V1oO2g6- and V10028H5- solution species (9), using a non-porous (200 m2g-I) y-alumina support. Aluminum niobate supported vanadium oxides have been prepared by grafting VOCl3 in inert atmosphere on AlNbO oxides calcinated at different temperatures. The AlNbO oxides have been prepared from oxalates precursors (7). All catalysts have been calcinated under air flow at 500OC. The catalysts have been tested with propane oxidative dehydrogenation in a flow system. The catalyst (100-350 mg) was deposited on a fixed bed in a quartz microreactor (Utube, 13 mm diameter) operating under atmospheric pressure. The gas mixture containing propane (2 vol %), 0 2 (19.6 vol %) and N2 (78.4 vol %) was fed at a flow rate of 50 ml/min. The conditions for on-line chromatographic analysis have been described elsewhere (8). The vanadium coordination was investigated at ambient conditions by UV-visible, Raman, V NMR and ESR spectroscopies after preparation and catalytic testing. In situ characterization of gamma-alumina supported vanadium oxides has also been performed in a Raman cell under dry air.
3. RESULTS 3.1. Characterization after calcination Table 1 summarizes the results of chemical analysis of the two families of catalysts. It must be noted that the AlNbO supports in samples AN500, AN600 and AN650 are amorphous to Xray diffraction. Sample AN750 corresponds to crystallized AlNbO4. It is seen that the vanadium superficial coverage on the four AlNbO supports varies within a narrow range (0.18 to 0.26), well below the values obtained on y-Al2O3 (0.28 to 0.65). This has been associated with the low density of surface hydroxyl groups on AlNbO oxides when compared to y-Al2O3.
85
UV-visible DRS spectra of the catalysts after calcination show a strong asymmetrical band in the 350-420 nm domain. The position of the band for all solids is reported in table 1. This range is characteristic of the ligand-metal charge transfer observed with V5+ ions. No signal is observed in the 700-800 nm zone, corresponding to the V4+ d-d transition. It is difficult, on the basis of UV experiments to assign a definite coordination to the superficial V species, as the spectrum is not related to that of bulk reference compounds with tetrahedral and octahedral coordination, Table 1 Comparison of some features of the two catalysts series Catalyst
BET area (m2g-1)
[V](at/nm2)
8
h,,,,
AN500 AN600 AN650 AN750
79 73 57 39
1.6 1.4 1.1 1.5
0.26 0.23 0.18 0.25
393 396 396 400
A2
200 200 200
3.9 3.3 1.7
0.65 0.54 0.28
420 3 80 350
A4 A7
G(ppm) -580, -548, -541, -486,
-1984 -1915 -1911 -1903 -502 -530 -544
AN: VO,/AlNbO. The associated number in the name indicates the calcination temperature of the support. A: VOxly-Al2O3. The number indicates the pH of the adsorption solution. [V]: vanadium superficial density. 8: vanadium superficial coverage (calculated from the unit area: A(VO,)=0.165 nm2 ). h,,,: position of UV-visible band. 6(pprn):S1V NMR chemical shift in reference to VOC13.
Table 1 shows also the 51V N M R chemical shift, in reference to VOC13, observed in the two families after calcination. On the basis of the anisotropy of the N M R signal, the sharp line observed (7) at -1900 ppm for the vanadium oxide species supported by AlNbO oxides has been attributed to an isolated vanadate with tetrahedral coordination. The wide, asymmetrical band, observed at approximately -500 ppm is associated with V species with distorted tetrahedral coordination. In contrast with the former, it is assumed that these species are condensed vanadates, forming linear chains or bidimensional arrays. It is seen that the proportion of the two kinds of tetrahedral vanadates (isolated or condensed) on AlNbO supports, depends on the calcination temperature of the material. A Raman scattering study was only possible for VOx/y-A1203 samples, because of a strong absorption by the Nb-0 vibration modes of the AlNbO support in the same region as the V-0 bands, As observed by other authors (lo), the nature of the alumina supported vanadates is influenced by the water pressure above the sample. The spectra have thus been collected at ambient temperature before and after in situ calcination (400OC under dry air). Only results after dehydration are shown in fig. 1.
86
.-a C
fa
800
1000 cm-1
1200
Figure 1. Raman spectra of VOx/y-A1203 (A2, A4 and A7) and alumina (A) A wide band is observed in the three samples A7, A4 and A2 in humid and dry conditions in the 960-995 cm-l domain; after drying, a shoulder develops above 1000 cm-l. This can be attributed to a V=O species forming three bonds with the support: this species is not stable in presence of water, forming hydroxylated, mono-oxo species which absorb in the same region as polycondensed vanadates (960-995 cm-1). W e therefore conclude that both types of vanadates (isolated and condensed mono-oxo vanadates) are present at the surface of the alumina support in the conditions of the catalytic reaction. The shift of the band to lower frequencies from A2 to A7, in hydrated samples, has been interpreted (1 1 ) as a decrease in lateral interactions between linear or branched vanadate chains as their superficial density decreases. 3.2. Propane oxydehydrogenation
Catalytic results for propane oxidative dehydrogenation at approximately isoconversion are shown in table 2.
Table 2 Catalytic results for propane oxydehydrogenation Catalyst
Mass (mg) T(0C) Conversion (“A) TON(s-1)
A2 A4 A7
100
100
100
350 350 350
13.5 8.8 <1.
A7
100
400
7.5
AN500 AN600 AN650 AN750
200 200 350 125
500 500 500 500
11.8 9.2 10.4 11.5
7 . 7 lo4 ~ 6 . 0 lo4 ~ <1.3 x lo4
2.1 x 2.0 10-3 2.1 x lo-’ 7.0 10-3
C?Hh
Selectivity(“) co CO:,
c
41 50.4
--
17.5 14 --
28 16
__
-_
58.4
12.1
--
70
12 65 62 44
87 2 16 18
__
99 100 99 87
33 21 25
87 80
C= selectivity sum (carbon balance); TON= turn over number (propane molecules transformed per vanadium atom and per second) The two families of catalysts present different behavior. VOx/y-A1203 catalysts are much more active than VO,/AlNbO catalysts so that it has not been possible to compare the two series at the same temperature. Results for sample A7 are shown at 400oC because practically no conversion is observed at 350oC. This sample is thus far less active than A4 or A2.This point is more obvious from their turnover numbers (TON). The propene selectivity pattern is however consistent for the three samples (1 1). VO,/AINbO catalysts on the other hand present a higher selectivity to propene at an intermediate calcination temperature of the support (AN600 and AN650). Sample AN500 shows a very low propene selectivity and a higher C 0 2 selectivity. Sample AN750 shows a selectivity compatible with alumina-supported vanadates; its activity (TON) is also the highest in the VOx/AINbO series. 3.3. Characterization of VO,/y-A1203 after catalytic testing
The UV-visible DRS spectra performed on alumina supported vanadium oxides show a new band at 700-800 nin which is attributed to the d-d transition of V4+ ions in octahedral symmetry. It is observed that the intensity of the band increases strongly with the vanadium coverage. On the other hand, the charge transfer band at 300-400 nm has not been significantly modified when compared with the calcinated catalyst. 51V NMR spectra of the same solids are also very similar before and after reaction. ESR spectra measured at ambient temperature show identical qualitative features for the three catalysts, but with different intensities. The ESR parameters are reported in table 3, where they are compared to the parameters of reduced amorphous V2O5 obtained from (12). A reasonable fit is observed between the two sets of parameter. It is interesting to note that the structure of amorphous V2O5 has been proposed to be built up from branched tetrahedrally coordinated mono-oxo vanadates (12).
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Table 3 Spin Hamiltonian Parameters Spin parameters
VOX/y-Al203(after testing) 1.919 1.982 190 Gauss 70 Gauss
amorphous V205( 12) 1.913 1.985 176 Gauss 66 Gauss
4. DISCUSSION 4.1. VOx/y-A1203 UV-visible spectroscopy clearly establishes the presence of only V5+ species dispersed at the surface of the support after the calcination of the catalysts. l V N M R spectroscopy shows that the major part of vanadium ions are tetrahedrally coordinated and form linear chains or bidimensional arrays. Thus, the two techniques do not show any clear difference between the three samples. Raman spectroscopy indicates that mono-oxo polyvanadates are formed with a higher degree of lateral interaction as superficial coverage increases. Tables 1 and 2 show that the turnover number for these solids strongly increases with their vanadium content. This observation suggests that dispersed species are not very active sites for propane oxydehydrogenation. As indicated by Raman spectroscopy, the extent of lateral interaction between vanadate chains is the only factor differentiating the three catalysts after calcination so that higher activity of vanadate species might result from these interactions. UV-visible and 51V NMR spectroscopy show that little alteration occurs in the vanadium V structure after partial reduction. These results on one hand, and the reasonable similarity of the ESR spectra in reduced VOx/y-A1203 catalysts and reduced amorphous V2O5, on the other hand, lead us to suggest that the active site in propane oxidative dehydrogenation is part of a branched vanadate chain in which vanadium is tetrahedrally coordinated. 4.2. VOx/AINb04 Two different vanadium environments have been characterized by 51V N M R spectroscopy. The first one is preferentially observed when the AlNbO support is calcinated at low temperatures and is characteristic of condensed tetrahedrally coordinated vanadium sites observed on y-Al2O3. As the calcination temperature of the support is increased, this species is substituted by a more symmetrical one which has been assumed to correspond to an isolated vanadium site. Table 2 shows that the turnover number is approximately the same in the three samples, AN500, AN600 and AN650. AN750 is roughly three times more active. These results show that the symmetrical (isolated) vanadate on AlNbO4 is more active than the distorted (condensed) species on amorphous AlNbO oxides. Propene selectivity on the other hand is the
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best at intermediate calcination temperature (i.e. amorphous AlNbO oxides with reduced superficial hydroxyl concentration). At lower calcination of the support ( A N S O O ) the catalyst gives very poor propene selectivity. The activity and selectivity pattern of these AlNbO supported vanadates is thus not clear. 4.3. Comparison of t h e two systems
As too many parameters are involved in the comparison of the two series (nature and surface area of the support, vanadium loading, reaction temperature) this will be restricted to the analysis of the selectivity differences. Indeed, the curve selectivity-conversion for VOx/yA203 catalysts is not significantly affected by vanadium loading and reaction temperature (1 1). Only two variables will thus be considered to analyze the catalytic results summed up in table 2. These are the degree of vanadium condensation and the chemical properties of the support. The vanadate species supported by amorphous AlNbO oxides may be compared to the alumina supported vanadates as their 51V NMR spectra are quite similar. W e have concluded for VOx/y-A1203 solids that an increase in lateral interactions leads to more active catalysts for propane oxidative dehydrogenation. Thus, it is not easy to understand, on this basis, why the new symmetrical (isolated) species, formed on the crystallized AlNbO4 support, should be more active than the distorted (condensed) one formed on amorphous AlNbO supports. We therefore must imagine that redox properties of the vanadate species depend on the bonds formed with the support. In a previous work (7), we have determined the acidity of the hydroxyl groups as a function of the calcination temperature of the support, by measuring their turnover number for isopropanol dehydration. It appeared that OH groups on crystallized AlNbO4 oxide are much more acidic than corresponding amorphous AlNbO-bonded groups. It therefore seems reasonable to admit that some field effect, induced by the linkage of niobium to aluminum sites, should also modify the electronic distribution in the V - 0 bond. It is obvious that some modification occurs from the 51V NMR spectrum: indeed the strong chemical shift (-1900 ppm) characterizes an unusual electronic density around the vanadium nucleus. It is also interesting to note that the basicity of the vanadium bonded oxygen, measured by oxidation of isopropanol to acetone, increases with the concentration of the new species. Also very striking is the lack of selectivity observed in sample AN500. If we assume that the vanadate species are the same as in the catalyst AN600, and VOx/y-A1203, as indicated by l V NMR spectroscopy, we must imagine that some influence from the support modifies the catalytic properties of the species. It might be possible that acidic functions of the support coupled with redox sites, for example, strongly influence the selectivity of the catalyst. In conclusion, the comparison between the two families of catalysts reported in this work suggests that the reactivity of supported vanadium oxides is determined by the degree of condensation of the vanadate species and the acid-base properties of the support. It seems that the challenge in controlling the catalyst reactivity will be mainly resolved when both steps, support preparation and vanadium dispersion, are strictly controlled to yield a strongly homogeneous and well defined superficial structure. REFERENCES 1. D.Siew Hew Sam, V.Soenen and J.C. Volta, J. Cata1.,123, 417 (1988) 2. A. Guerrero-Ruiz, I. Rodriguez-Ramos, J. L. G. Fierro, V. Soenen, J. M. Hermann and
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3. 4. 5.
6. 7.
8. 9. 10. 11. 12
J.C. Volta, in New Developments in Selective Oxidation by Heterogeneous Catalysis, in Surface Sciences and Catalysis, vo1.72, p203, P. Ruiz and B. Delmon Eds., Elsevier(l992) J.Haber, A.Kozlowska and R.Kozlowski, J.Catal., 102, 52 (1986) I.E.Wachs, J.Catal.,124,570(1990), U.Scharf, M. Schraml-Marth, A. Wokaun and A. Baiker, J. Chem. Soc. Faraday Trans, 8., 3299, (1991) J.M.Jehng and I.E.Wachs, Catal. lett.,13, 9, (1992) P.G.P.de Oliveira, F.Lefebvre, J.G.Eon and J.C.Volta, J. Chem. Soc., Chem. Com., 1480,2 1( 1990) P.G.P.de Oliveira, J.G.Eon and J.C.Volta, J.Cata1.,137, 257 (1992) M.T.Pope, Heteropoly and Isopoly Oxopolymetalates, Springer-Veda&( 1983) G.T.Went, S.T.Oyama and A.T.Bel1, J.Phys.Chem.,4240,94,(1990) , J.G.Eon, R.Olier and J.C.Volta, J. Catal., to be published A. Mosset, P. Lecante, J. Galy and J. Livage, Philos. Mag. (8) 46 (2), 137 (1982)
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J. EMBER (Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Krakow, Poland): Some time ago we have shown that the type of vanadium oxide species present at the surface depends on the water pressure and coverage. In the humid atmosphere decavanadate anions are favored, whereas on careful outgasing either isolated V04 tetrahedra appear or chains of these tetrahedra forming a metavanadate type structure. Which of these exist in the conditions of your oxydehydrogenation and which is then an active phase?
J. G. EON (Instituto de Quimica, Rio de Janeiro, Brazil): We have shown (11) that decavanadate anions are not stable on the alumina surface. After calcination at 5OO0C, even in humid atmosphere, we have characterized mainly tetracoordinated species forming a chain or array structure. Isolated V04 species might coexist; as the specific activity increases greatly with vanadium concentration from A7 to A2, we suggest this species is not the active one on alumina supports. G. CENT1 (Dip Chimica Industriale e dei Materiali, V. Le Risorgimento , Bologna, Italy): In the samples prepared by supporting vanadium on alumina you observe an increase in the activity in the A7-A2 series that was attributed to an increase in the lateral interaction between vanadate species. However, these samples have a different vanadium content. My question is therefore how change the specific activity per vanadium site in these samples and how this specific activity can be correlated to the change of surface species of vanadium. J. G. EON: What we observe is indeed an increase in specific activity per vanadium site (turnover number) in the series A7, A4, A2. We suggest that lateral interactions between vanadate chains stabilize the superficial vanadyl formed after reduction by propane (1 1).
J. S. R U E (Inst. Catalisis, Madrid, Spain): The table with the ESR results is not well written. The g-values should appear as g,, and gL. They should be determined by computer simulation. The hyperfine splitting constants indicate that the sites are different, not similar and should also differentiate between the perpendicular and parallel components. J. G. EON: We agree with these comments. However, the ESR parameters of V4+ in alumina supported vanadates are very different from the values observed in crystallized V2O5 or VOPO4 and strikingly in reasonable agreement with those of amorphous V2O5. Although the sets are clearly not identical, we use the spectrum as a finger-print, suggesting that the structure of vanadate layers on alumina should be compared to that of amorphous V2O5 where vanadium is also tetracoordinated.
G. BUSCA (Instituto di Chimica, Faculta di Inorganica, Universita di Genova, Genova, Italy): In your lecture you denoted the active sites on VOx/A1203 as tetrahedral with Td symmetry. Previous data from several laboratories agree showing that active sites on vanadia-alumina in dry conditions are tetracoordinated but not tetrahedral. Vibrational spectroscopies in fact show that these species have one short V=O bond and probably, three
92
longer V - 0 bonds. They are consequently tetracoordinated, but with C3v symmetry. Tetrahedral centers (Td symmetry) are instead typical of vanadate species in basic environments. In your opinion, your data support a tetrahedral (Td) symmetry or a tetracoordinated symmetry (CjV)?
J. G. EON: We think it is impossible to attribute symmetry labels from our data to alumina supported vanadates. Our spectroscopic data show that these species are mono-0x0, tetracoordinated vanadates. J. F. BRAZDlL (BP Chemicals, Warrensville Research Center, Cleveland, Ohio, USA): Since aluminum niobate crystallizes with the rutile structure and vanadium is known to dissolve readily into rutile structures, does the isolated vanadium species you see arise from formation of solid solution between vanadium and aluminum niobate? J. G. EON: This is an interesting point. However, aluminum niobate does not crystallize with the rutile structure but is isomorphic to Ti02(B)(7). Moreover, vanadium dissolves in rutile as the reduced V4+ cation. Since we have observed only V5+ species by UV-visible spectroscopy, we did not consider this possibility.